you see,my mum promised to buy me --- MP4 for my birthday.

you see,my mum promised to buy me ___ MP4 for my birthday.---oh,really?but how i wish i could listen to music____MP3 a:an ;on b:a ;over c:a ;by d:an;with 请说明为什么选的原因和其他选项不可以的原因,